1. Technical Field
This invention relates to a servomotor velocity control apparatus forming a digital servo system, the gain of which is capable of being varied.
2. Background Art
In an ordinary digital servo system, the torque command of a servomotor is set based on an error signal between a commanded velocity and an actual velocity, and a pulse coder generally is used as velocity detecting means for detecting the actual velocity.
To accurately control the velocity and position of the servomotor based on the error signal, a PI compensating circuit is usually required for performing a proportional operation and an integrating operation. In order to minimize the influence of frictional resistance in a servo system which includes such a compensating circuit, consideration has been given to altering the proportional gain and integration gain of the compensating circuit in dependence upon the velocity of the servomotor.
With this conventional servo system, it becomes necessary to suitably alter the integration gain when it is attempted to maintain positional accuracy by removing the influence of external disturbances. However, when the motor is operating at a low velocity, stabilized operation of the feedback system is lost. The reason for this is that the actual velocity signal from the velocity detecting means such as the pulse coder is fed back in discrete fashion; hence, velocity detection resolution declines at low rotational speeds.
More specifically, when the resolution of detected velocity declines, an offset due to static frictional resistance develops in the servo system when it is operating at very low velocity. Consequently, even if an attempt is made to raise positioning accuracy by changing the integration gain based on the actual velocity signal fed back from the pulse coder, the gain cannot be altered by making a proper evaluation of the amount of offset. An irregularity develops in the rotational velocity of the servomotor as a result.